1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) panel capable of reducing a persistence degree and a development method thereof.
2. Description of the Related Art
Each of FIGS. 31 and 32 is a schematic sectional view showing a structure of one pixel of an LC panel. FIG. 31 shows a state where no voltage is applied, and FIG. 32 shows a state where a voltage is applied.
The LCD panel includes substrates 10 and 20 opposing to each other, and a sealed-in nematic liquid crystal 30 having an anisotropic dielectric positive constant. In the substrate 10, a flat electrode 12, a dielectric layer 13 and a vertically oriented layer 14 are formed on a face of a transparent insulating substrate 11, for example, a glass substrate, and on the other face thereof, a polarizer 15 is formed. In the substrate 20, a common electrode 23 is formed on one face of a transparent substrate 21, for example, a glass substrate, an insulating layer 24 is formed thereon, a pixel electrode 25 is formed on the insulating layer 24, and further an insulating layer 26 and a vertically orientated later 27 are formed thereon. On the other face of the substrate 21, a polarizer 28 is formed. Transmission axes of the polarizers 15 and 28 perpendicularly cross over each other.
When backlight in the direction shown by arrows in FIG. 31 enters into the LCD panel, the light is transformed into linearly polarized light by the polarizer 28. When the flat electrode 12, the common electrodes 23 and the pixel electrode have the same potential, the liquid crystal 30 effects no change in the plane of polarization of the linearly polarized light, and therefore the linearly polarized light cannot be transmitted through the polarizer 15, resulting in a dark state.
When, as shown in FIG. 32, the flat electrode 12 and the common electrode 23 has the same potential but the pixel electrode 25 is applied with a potential different from the both former electrodes, an electric field arises. Dotted lines of FIG. 32 show the lines of electric force. Liquid crystal molecules are inclined relative to an incident light direction under influence of the electric field to cause birefringence, and part of the light can transmit through the polarizer 15, resulting in a bright state.
Since the common electrode 23 and the pixel electrode 25 are made of an opaque metal, behaviors of liquid crystal molecules over the electrodes are not problematic in terms of display.
If the flat electrode 12 does not exist, liquid crystal molecules between the pixel electrode and the common electrode 23 tend to reduce inclination thereof, which will produces the drop region of transmittance. The flat electrode 12 makes the electric field between the common electrode 23 and the pixel electrode 25 asymmetric so as to contributes to prevent the transmittance from locally dropping. The dielectric layer 13 reinforces the lateral component of the electric field in the liquid crystal 30 to make it possible for the liquid crystal 30 to be driven with lower applied voltage. The common electrode 23 and the pixel electrode 25 each are stripe electrodes extending in the direction perpendicular to the sheet of FIG. 32, and alternately formed on the top and bottom surfaces of the insulting layer 24. The insulating layer 24 is for preventing common electrodes and pixel electrodes from short-circuiting at positions where the both overlap as will be described later. The insulating layer 26 is for reducing the persistence degree.
FIG. 33 shows an electrode pattern of one pixel, formed in the substrate 20 of FIG. 31. FIGS. 34 and 35 are patterns of the pixel electrode 25 and the common electrode 23, respectively, of FIG. 33.
In FIG. 33, a data line DL1 and a scan line SL1 cross over each other with an insulating layer interposing therebetween. Each of the pixel electrode 25 and the common electrode 23 has a stripe section and a peripheral section connecting ends of the stripe section. The lines of the stripe section are inclined 45 degrees to each of the scan line SL1 and the data line DL1.
When the potential of the scan line SL1 goes high, a TFT 29 is turned on to apply the potential of the data line DL1 onto the pixel electrode 25 and generate an electric field between the stripe electrodes of the pixel electrode 25 and the common electrode 23. The longitudinal direction of the upper half of the stripe electrodes is different from that of the lower half of the stripe electrodes by 90 degrees as shown in FIG. 33, whereby the LCD panel has wider range of viewing angles than in a case where the both halves of the stripe electrodes are parallel to each other.
The common electrode 23A has peripheral protrusions which are connected to the common electrodes of adjacent pixels not shown.
FIG. 36(A) is an enlarged partial view near a crossover of a stripe electrode and the peripheral section of FIG. 33. FIG. 36(B) shows the lines of electric force with dotted lines near the crossover when a voltage is applied between the pixel electrode 25 and the common electrode 23.
A peripheral section of the pixel or common electrode has crossover portions to stripe electrodes of the common or pixel electrodes with the insulating layer interposing therebetween since a pixel has a rectangular shape, and each of the pixel electrode 25 and the common electrode 23 has stripe electrodes in parallel to each other and has a continuous shape. For example, a side 251 of the pixel electrode 25 is connected to a side 252 of the peripheral section, and a side 231 of the common electrode 23 is parallel to the side 251, while the side 231 crosses over the side 252 at an acute angle.
FIG. 37 is a schematic sectional view showing inclination of liquid crystal molecules between the pixel electrode 25 and the common electrode 23 of one pixel of the LCD panel when a voltage is applied therebetween.
In FIG. 32, a structure between the pixel electrode 25 and the liquid crystal 30 is different from that between the common electrode 23 and the liquid crystal 30, which causes persistence.
In FIG. 36(B), since the side 252 crosses over the side 231 at an acute angle, an electric field therebetween near the crossover is stronger than that between the parallel sides. Further, a direction of electric field strength near the crossover is different from that between the parallel sides. Due to such conditions, a transmittance-voltage characteristic near the crossover is different from that between the parallel portion, causing not only degradation of an image quality but also persistence.
In FIG. 37, since the insulating layer 26 exists above the pixel electrode 25, application of an electric field in this portion is useless and effective application of the electric field to the liquid crystal 30 is prevented. If the insulating layer 26 is omitted in order to solve this problem, it causes more persistence since the insulating resistance of the vertically oriented layer 27 is low. If the pixel electrode 25 is exposed to the liquid crystal 30, not only is the degree of persistence enhanced, but liquid crystal molecules also decompose. Further, since the top surface of a pixel electrode 25 is flat, it is not possible to effectively apply an electric field to the liquid crystal 30 in relation to transmittance, which prevents achieving higher contrast display.
In development of an LCD panel, measurement of a persistence degree is performed at each trial when a structure or material of the LCD panel is changed in order to reduce the persistence degree to a value lower than a given value, and it takes, for example, 48 hours to measure the persistence degree in each trial, which makes a development term thereof longer.
Accordingly, it is an object of the present invention to provide a liquid crystal display panel capable of reducing a persistence degree.
In one aspect of the present invention, there is provided a liquid crystal panel comprising: first and second substrate; and liquid crystal interposed between the first and second substrates; the first substrate comprising: an insulating substrate; first and second electrodes, formed over the insulating substrate, for a display voltage to be applied therebetween; and a first insulating layer covering the first and second electrodes; wherein the first electrode is disposed higher than the second electrode in relation to a direction from the insulating substrate toward the second substrate, and the first and second electrodes overlap each other with a second insulating layer being interposed therebetween at an overlapping portion, wherein a thickness of the first insulating layer on the first electrode is substantially equal to the insulating layer on the second electrode.
With this configuration, when the voltage signal is applied between the first and second electrodes, electric states over the first and second electrodes are almost the same, whereby persistence is reduced in comparison with a case where the thicknesses are different from each other as shown in FIG. 31
Other aspects, objects, and the advantages of the present invention will become apparent from the following detailed description taken in connection with the accompanying drawings.